Rival armed groups may have killed hundreds of civilians in massacres and other "incomprehensibly vicious" attacks in parts of the Democratic Republic of Congo, UN officials said.

The violence is focused in North Kivu, near the border with Rwanda, where warring groups have targeted villages seen as supporting their opponents, while the national army has been diverted to fight mutineers known as M23.

"The deterioration of the overall security situation in North Kivu following the M23 mutiny and related ruthless attacks against civilians is extremely alarming," said Roger Meece, representative of UN Secretary General Ban Ki-moon.

Mr Meece was cited in a UN report issued in Geneva as saying a new round of systematic killings of villagers appeared to have occurred in early August.

UN High Commissioner for Human Rights Navi Pillay said allegations of hundreds of killings were still being verified, but preliminary investigations suggested a large number of people, mainly women and children, had been slaughtered.

The Congo government in Kinshasa this month rejected calls by other countries in the region for an exclusively African force to tackle the insurgency.

Congo says some of the insurgent groups have support from countries such as Rwanda and Uganda and wants an expansion of the 17,000-member UN peacekeeping force.